Method and System for Determining the Delay of Digital Signals

ABSTRACT

A method of and system for determining the time required for a digital bit or bit stream to traverse a round-trip path from a source transceiver to at least one destination transceiver and back is disclosed. The relative timing of the transmitted bit or bit stream is compared to the return bit or bit stream using a high speed comparison configuration so as to provide in substantially real-time various measurements related to or derived from the time required to traverse the round trip path, including distance measurement in indoor positioning, real-time locating, adaptive cruise control, intelligent transportation systems, robotics, collision avoidance, personnel accountability, emergency location, search and rescue, loss and theft prevention, logistics, marine safety, network analysis, communication channel characterization, and other high-speed measurement tasks. In addition, a method of and system for determining the distance between a source transceiver and a destination transceiver, and a method of and system for determining the angular position of a transceiver with respect to at least two other transceivers, are disclosed.

RELATED APPLICATIONS

This application is a continuation of patent application Ser. No.11/139,063 filed on May 27, 2005 which claims priority to provisionalpatent application Ser. No. 60/574,914 filed May 27, 2004, both of whichare incorporated herein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to a method of determining thedelay time (or transit time, or propagation delay) of a digital signaltransmitted from a source and returned to that source, regardless of thetransmission medium. More specifically, a method of comparing atransmitted bit or bit stream to a returned bit or bit stream isdescribed, in which the comparison is performed primarily as a logicaloperation, or other high speed comparison, and the output of theoperation is a binary representation of the round trip time of the bitor bit stream.

BACKGROUND OF THE DISCLOSURE

There are many applications that require the determination of the timefor a digital bit or bit stream to traverse a round-trip path from asource transceiver to a destination transceiver and back. Suchdeterminations are helpful for example, for sensing the proximity of oneobject having one transceiver attached to it relative to another objecthaving the other transceiver attached to it. Application areas include,but are not limited to, distance measurement, indoor positioning,emergency location, search and rescue, personnel accountability systems,security systems, collision warning, adaptive cruise control,intelligent transportation systems, logistics, robotics, networkanalysis, communication channel characterization, and others.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to a system for and method ofdetermining the time required for a digital bit or bit stream totraverse a round-trip path from a source transceiver to a destinationtransceiver and back. The path may also include multiple destinations,as, for example, in a ‘multi-hop’, mesh, or an ad-hoc transmissionprotocol, or other transmission protocol. For the purposes of disclosingthe system and method, a round trip will be assumed to be from a sourcetransmitter to a particular destination and back to the source. However,the system and method may be applied for round-trip delay measurementswith any number of intermediate points.

One central feature or aspect of the disclosed system and method is ahigh speed comparison of the transmitted bit or bit stream to the returnbit or bit stream. Measurement error sources, such as fixed delays orjitter introduced by circuit components, are to a significant extentmeasured in real-time and used to correct the measurement during theprocess.

The disclosure further includes an improved system for and method ofdetermining the angular position of a transceiver with respect to othertransceivers by taking the distance measurements from a first and secondtransceiver and comparing them to the distance measurements obtainedfrom at least one other (third) transceiver.

The disclosure further includes a system for and method ofinterconnecting the transceivers to form a wireless communicationnetwork and coordinating the individual transceiver measurement andcommunication tasks through the network. Such coordination allows alarge number of transceivers to operate in a given area, without thecommunication conflicts that would typically arise in such situations.The network can also be used to continually refine the accuracy of themeasurements (distance and angular position), using a system and/ormethod disclosed in this application.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summarized system and method of the disclosure, thevarious features thereof, as well as the invention itself may be morefully understood from the following description when read together withthe accompanying drawings:

FIG. 1 is a graphical representation illustrating an example of alogical comparison of a transmitted and returned digital signal and theresulting binary output representing the delay.

FIG. 2 is a graphical representation illustrating an example of apreferred method of interpreting the binary output bits.

FIG. 3 is a graphical representation showing a plot of the output valuesof the leading edge byte and trailing edge byte as a function of signaldelay time.

FIG. 4 is a graphical representation showing a plot comparing measuredversus actual delay values for the leading and trailing edge bytes, asinterpreted using the method of the invention, in an ideal (noise-free)case.

FIG. 5 is a graphical representation illustrating the effects of jitteron the binary output values.

FIG. 6 is an exemplary table illustrating the corrected (processed)output in the presence of jitter.

FIG. 7 is a graphical representation illustrating the method of nestedbit patterns.

FIG. 8 is a schematic block diagram showing two identical transceivers,both operating on the same communication frequency (simplex).

FIG. 9 is a schematic block diagram showing a transceiver pair usingdifferent frequencies for send and receive (duplex).

FIG. 10 is a schematic block diagram of a transceiver pair withmodulation bypass, which permits near real-time measurement of delaysintroduced by components in the transceivers themselves.

FIG. 11 is a schematic block diagram of a reference pair oftransceivers, in which the source transceiver (e.g., Reader) isconnected to a network for communication with other system components.The second transceiver of the reference pair is assumed to be a small,portable device (e.g., Tag). A third transceiver, also for example aTag, is in an unknown location.

FIG. 12 is a schematic block diagram of a multiple-transceiver system,in which several readers are connected to each other via a wirelessnetwork, and illustrates the azimuth angles to be determined in additionto the distance to each Tag.

FIG. 13 illustrates the definitions used in the text for using thedisclosed methods to perform triangulation calculations and thusdetermine azimuth angle between transceivers.

DETAILED DESCRIPTION OF THE DRAWINGS

The present disclosure relates generally to a system for and method ofdetermining the delay time (or transit time, or propagation delay) of adigital signal transmitted from a source and returned to that source,regardless of the transmission medium. More specifically, a system forand method of comparing a transmitted bit or bit stream to a returnedbit or bit stream is described, in which the comparison is performedprimarily as a logical operation, or other high speed comparison, andthe output of the operation is a binary representation of the round triptime of the bit or bit stream. In accordance with one aspect of thedisclosure, the system and method described is preferably asynchronousso that clock synchronization between the signal source anddestination(s) is not necessary to obtain the measurement. Furthermore,in accordance with another aspect of the disclosure, a system and methodare presented which include a nesting of bit streams within longer bitstreams to allow the delay measurements to be made across several ordersof magnitude in time. This permits increasing the precision of themeasurements, without a corresponding increase in the time required forprocessing the signal over many samples, as is done in many prior artsystems. Systems and methods are also disclosed which allow a pluralityof transceivers to exchange distance measurements to determine theangular position of the transceivers relative to each other.

In accordance with one aspect of the present invention, a method andsystem are disclosed that are distinct from the prior art, in particularwith regard to RADAR systems. The disclosed system and method assumesthat the ‘target’ is a “cooperative” one, and has compatible transceivercircuitry that provides an active response. RADAR systems typically relyon reflected energy from the targets, which are generally not“cooperative”, especially those at very close range where measurementsare more difficult. This is an important aspect of the presentdisclosure, as the cooperative transceivers described herein areintended to enable communication between objects, vehicles, or livingthings, such as exchange of identification numbers, descriptive data,warnings, and other information. Such communication will serve manyuseful purposes, which RADAR systems alone cannot achieve. Furthermore,the disclosed system and method do not assume a radar system as a sourcetransceiver, nor do they demand the complex receiver electronics andsignal processing required in RADAR systems. Thus, the presentdisclosure is applicable as a geo-locating system for and method ofusing any transceiver type and any modulation/demodulation scheme,whether based on radio, other electromagnetic waves, or other wavepropagation phenomena. It is also applicable as a system for and methodof characterizing the delay of a digital signal as it passes through anycommunication path, or through multiple communication paths, over, forexample, wires, fiber optic-cables, bulk solids, liquids, gases, and soon. The application areas of the method and system are thus very wideranging, and go well beyond the scope of RADAR or similar systems.

The disclosed method and system is also distinct from the prior art withregard to Radio Frequency Identification (RFID) systems, and Real-TimeLocating Systems (RTLS). RFID systems typically make use of multipletransceivers, but are limited to one transceiver (a Reader) determiningif any target transceivers (tags) are present, and reading the dataencoded within them. They perform a presence detection andidentification function, but do not obtain genuine distance information.Real-Time Locating Systems, on the other hand, are designed to providedistance and azimuth information (localization) for a large number oftransceivers, in addition to identification information. However, priorart RTLS systems have significant limitations, which the disclosedsystem addresses. For example, the Global Positioning System (GPS)provides three-dimensional localization of suitably equippedtransceivers, but requires processing of data received from at leastthree orbiting GPS satellites. Applications where satellite visibilityis poor or unavailable, such as indoors, in underground mines, etc.,cannot make use of GPS directly. Other RTLS systems, especially thosedesigned for use indoors, utilize infrared, ultrasound, and wirelesscommunications protocols (e.g. IEEE 802.11x), often in combination, butare limited either by the requirement for line-of-sight communication,signal processing complexity resulting in slow performance, high systemcost, inadequate spatial resolution, or a combination of theselimitations. By providing digital signal delay information rapidly andinexpensively, and utilizing signal detection and comparison systems andmethods that can operate independently of the particular transmissionmedium, method or protocol, the disclosed system and method addressesthese limitations.

Embodiments of the disclosed system and method will typically includethe use of at least two transceivers and a minimal number of peripheralcomponents, such as power sources, antennas or other transmission mediainterfaces, visual and audible indicators or alarms, user controls, etc.Transceivers may be mobile or fixed, and powered externally or withinternal power sources such as batteries. Owing to the simplicity of theapproach, the transceivers may be small, battery powered, and madecompatible with various packaging methods suitable for wearing by humansor animals. They may also be mounted to vehicles, machines, structures,shipping containers, pallets, and product packaging. For theillustrative purposes of describing the system and method, transceiverswill be referred to as ‘source transceivers’ or ‘destinationtransceivers’ with the ‘source transceiver’ being the device initiatingthe measurement, and the “destination transceiver” receiving theinitiating measurement signal from the “source transceiver”. However,any given device might perform either function at a given time.

The source transceivers produce interrogation signals, to which thedestination transceivers respond. The source transceivers can measurethe time taken for a digital signal to traverse from the sourcetransceiver to the destination, and back again. One aspect of the basicmethod and system is illustrated in FIG. 1. The source transmittergenerates a bit or bit stream, indicated in the drawing as TransmittedBit, at time T₀=0. The period of the Transmitted Bit is T₂ seconds long(the time T will be measured in convenient units, such a milliseconds,microseconds, nanoseconds, etc.). The signal will travel to thedestination transceiver where it is re-transmitted, and finally returnedto the source transceiver, as the Return Bit. There will be a time delayof T_(i) seconds between the departure time of the original signal andthe arrival time of the return signal, and this delay time is related tothe distance the signal traveled, the propagation speed through thetransmission medium, and the propagation speed of the signal through thetransceiver components. T₁ may be larger or smaller than T₂, though inthe figure it is shown for illustrative purposes as smaller since thesemeasurements, with methods and systems found in the prior art, aretypically the most difficult to make. In cases where a signal passesthrough a number of intermediate transceivers, which may re-transmit thesignal through different media (e.g., water, air, building materials),the propagation speed in each medium must be known for each leg of thetransmission path (electromagnetic, ultrasonic, infrared, acoustic,etc.).

The ‘High Speed Comparison’ Function

A central element of the present disclosure is the use of a high speedcomparison circuit to detect the departure and arrival times of thesignal bit or bit streams. This circuit may be composed of logic gates,or combinations thereof, including counters, timers, and othercomponents commonly used in high speed digital processing, but may alsoconsist of analog or mixed-signal components such as sample-and-holdcircuits, analog-to-digital converters, edge detection circuits, and thelike. The primary function of the circuit, regardless of theimplementation, is to perform a comparison of the transmitted andreceived signals, the output of which is a value proportional to theirrelative separation in time.

As shown in FIG. 1 the departure time is indicated at T₀, while thearrival time is T₁. Referring again to FIG. 1, the return signal arriveswith some unknown delay (indicated as T₁-T₀ in FIG. 1). While thetransmitted bit is generated with a certain time base, or clockfrequency, the return bit is interpreted by a circuit with a clockfrequency substantially higher that the transmit clock. In the figure,the return bit is evaluated, or ‘over-sampled’ at, for example, 8 timesthe clock frequency of the transmitted bit. That is, in the examplegiven each transmitted bit of length T₂ (T₂-T₀ with T₀=0) produces eightreturn bits of length T₂/8 for each bit transmitted. In general theduration of the bits produced in the comparison circuit is LB=T₂/S,where S is defined as the over-sampling factor, and is also equal to thenumber of bits into which the return bit is divided, or S=T₂/LB. Otherratios can be implemented (e.g., S=16 or S=32), as can systems withvarying values of transmitted bit length, with obvious tradeoffs, forexample, for complexity, measurement resolution, circuit cost, and soon.

In a typical implementation, the transmitted bit and the return bit willboth be sampled at a multiple of the transmit clock frequency, and theresulting outputs compared using high speed logical comparisonconfigurations, such as high speed logical gates, e.g., XOR, OR, AND, ortheir equivalents, so that the results can be provided in substantiallyreal time. Other implementations of high speed comparison configurationscan be designed using, for example, edge detection circuits, timers,counters, sample-and-hold circuits, analog-to-digital converters, or acombination of these elements. Generally these components can beoperated at much higher frequencies than the transceiver processors, andcan be obtained at much lower cost, thus the system has advantages oversystems that require more intensive signal processing.

In the present disclosed system and method, the binary output bitsresulting from the over-sampling of the return bit are interpreted asbytes. As illustrated in FIG. 2, the Oversampled Return digital signalof FIG. 1 is represented as a digital signal having two 8-bit bytes,since the transmit bit is, in the example given, over-sampled by afactor of eight (S=8). The first byte corresponds to the time periodduring which the leading edge of the Return Bit arrives, and the secondbyte corresponds to the time period during which the trailing edge ofthe Return Bit arrives. If the transmit bit were over-sampled by 16,there would be four 8-bit bytes, and a corresponding increase in thetemporal resolution of the measurement. Generally, the duration ofLeading Edge (LE) and Trailing Edge (TE) time periods will vary from oneimplementation to another, depending on transmit bit or bit streamlength and the delay times anticipated for the measurement.

Note that, as best seen in FIG. 2, the disclosed system and methodarbitrarily assign the least significant bit (LSB) position to the firsttime slot in the leading edge byte of the Oversampled Return, while themost significant bit (MSB) is assigned to the last time slot. This isreversed for the trailing edge byte of the Return Bit, as this choicesimplifies the calculations that follow. Depending on the designers'preference, LSB and MSB for each byte can be reversed or not, since thebytes ultimately will carry the same information, albeit transformedmathematically. A novel and important aspect of the present invention isthat both the leading and trailing edges of the Return Bit are detectedindependently, providing two measurements of the arrival time of thereturn bit, or each bit in the return bit stream. These two measurementscan be processed, compared, and otherwise evaluated in numerous ways. Anexample of one such evaluation, providing a measurement method withnoise reduction, is given below.

In one embodiment, the binary output is produced by the application ofthe logical OR and “Exclusive OR” (or XOR) functions to the return andtransmitted bytes, as disclosed by way of example, in the followingparagraphs. Various decimal and binary representations of the output,including the number of bits in a byte representing the delay, can beused in the process, with suitable conversion or mathematicaltransformation from one to the other. In any case, the output will beconverted into a delay measurement available for practical use. Toaccomplish this, it is necessary to convert the binary or decimal valuesto units of T₂/LB or bits. Bits are easily related to units of time,since each bit has a known duration related to the clock frequency ofthe logical comparison configuration. For example, in a system using a 1GHz clock and 50% duty cycle to over-sample the return bits, one bitperiod equals 50% of the clock period, or 0.5 nanoseconds.

As discussed above, each comparison captures the departure of one bit(or bit stream) and the arrival of another, and over-samples the bit bya factor S. In each case, both the leading edge (LE) and trailing edge(TE) of each received bit are compared to the transmitted bit pattern orto other known values. Two measurements are thus generated per returnbit, which take the form of two digital bytes of S bits each. Theassignment of least significant bit (LSB) and most significant bit (MSB)is arbitrary, but for the purposes of disclosing the method, in theillustrated example it is assumed that the LSB arrives first for the LEbyte and the MSB arrives first for the TE byte, as shown in FIG. 2.

Measurement of Signal Delay with Noise Reduction Method

Typically, the output of the comparison will be in binary form, sincethe delay is represented by a group of bits. Referring again to FIG. 1,the Return Bit will have a leading edge that arrives at time T₁, and atrailing edge that arrives at time T₃. Ideally, the duration of thereturn bit would be a known quantity, T₂=T₃-T₁, but in practice thereare noise sources which introduce arrival time variations in both theleading and trailing edges of the bit. This noise, often referred to asjitter or phase noise, can have the effect of shortening or lengtheningthe bit, or of making the arrival time appear to vary, or both. Jitterwill generally be present regardless of the design of the transceivers,the choice of transmission medium, or other factors under the designer'scontrol.

To address this problem, one aspect of the disclosed system and methodcombines the logical comparison method and a novel interpretation of thebinary output, as illustrated in FIG. 2, with a novel method of reducingthe effects of noise in the signals.

FIG. 3 shows the values of the leading and trailing edge bytes(converted into their decimal equivalents) as a function of the signaldelay, given in bits, or units of T₂/LB. For example, with a delay ofonly one bit, the binary value of the leading edge byte will be 01111111 (LSB to MSB). The binary value of the trailing edge byte will be1000 0000 (MSB to LSB). The values represented by these bytes, whenconverted to the decimal system, are LE_(DEC)=254 and TE_(DEC)=128,respectively. Here, each 8-bit word has 256 allowable values, rangingfrom 0 to 255. The graph of FIG. 3 plots the values of the bytes as thedelay increases, and they are seen to have the expected base-2logarithmic shape.

For methods and systems that do not need to operate in the presence ofsignificant noise, a simple counting of binary output bits, or theirdecimal equivalents, may suffice. FIG. 4 illustrates the results whenthe LE and TE bytes are converted from decimal values into bits in theabsence of noise, and how these measured values correspond directly tothe actual signal delay. However, for practical methods and systems,phase noise in the return signal can present significant problems, andis a major reason that prior art tends to avoid reliance on simplerising- or falling-edge timing to determine signal delays. The effectscan be seen readily in FIG. 5, which shows an example of the decimalvalues of leading edge and trailing edge bytes, with a single bit of‘noise’ added to each. The noise manifests itself as an increase ordecrease in the binary output value, and the magnitude of the increaseor decrease varies depending on the ‘significance’ of the bitrepresenting the delay. This depends on the actual delay time of thesignal, so that the noise level at the output is a function of thesignal delay as well as the variation in arrival times of the edges.

FIG. 6 shows an example table of values (in binary and decimal forms) ofthe LE and TE bytes as the noise levels and actual signal delay vary.The variation of the value of the LE byte is a measure of the delay ofthe leading edge of the bit, plus variations in the LE arrival time dueto phase noise. Similarly, the variation of the value of the TE byte isa measure of the delay of the trailing edge of the bit, plus variationsdue to phase noise. For simplicity in the table, actual delays have beenapplied to both edges, and out-of-phase noise has been added to only theLE byte, but the method effectively sums the LE and TE bytes, so thatthe effect in the output would be no different even if noise were addedrandomly to both edges.

Jitter, or phase noise, can be thought of as having two components. Thefirst is an in-phase component, which either delays or advances both theleading and trailing edges of the bit. This appears as either increasedor decreased delay of the entire bit, and is difficult to distinguishfrom actual signal delay unless averaged over time or viewed in thefrequency domain, to allow only those variations that correlate withknown physical limitations of the transceivers and the change of theirrelative positions in time. The second is an out-of-phase component,which affects one edge with respect to the other, and appears as anincrease or decrease of the duration of the return bit. Prior artmethods which attempt to observe either the leading or trailing edge ofreceived bits are limited by the effects of both these noise components.As disclosed below, in accordance with one aspect of the presentinvention, a system for and method of substantially removing theout-of-phase component is provided.

For illustrative purposes, the logical inverse of the LE byte is alsolisted in FIG. 6 (denoted by /LE), since in the absence of noise thisterm will be identical to the TE byte. In fact, the difference between/LE and TE is a measure of the variation of pulse width in the signal,compared with the transmitted bit (in other words, the out-of-phasenoise component).

Applying the logical function “Exclusive OR” (XOR) to each byte and itsfull scale value (all ones in binary, or 2S in decimal), in a bit-by-bitfashion, results in values that can be used by the circuitry to subtractthe out-of-phase noise from the measured signals. The XOR function isapplied to the LE byte and 2^(S), and again to the TE byte and 2^(S),and the results are given the following physical interpretations:

/LE′ _(BIN)=XOR{2^(s) _(BIN) ,LE}=LE delay+noise

TE′ _(BIN)=XOR{2^(s) _(BIN) ,/TE}=TE delay+noise

The sum of these terms is proportional to the out-of-phase noise on bothedges plus two times the signal of interest, and is defined as:

Sum_(TE/LE)=OR{/LE′_(BIN),TE′_(BIN})

Note that the highest speed implementations of the system and methodwill likely use a high speed comparison configuration, such as logicgates that are devoted to this and other logical operations, rather thanusing general purpose processors. For purposes of disclosure of themethod and system, the decimal values are shown in FIG. 6 along with thebinary values, and then are converted into bits for the remainingcalculations. The decimal values are converted into bits according tothe following relations:

/LE′ _(BITS)=LOG₂{2^(S) −LE _(DECIMAL)}

TE′ _(BITS)=LOG₂{2^(S) −/TE _(DECIMAL)}

Sum_(BITS)=LOG₂ {/LE _(DECIMAL) +TE _(DECIMAL)+1}

Since Sum_(BITS) (and equivalently Sum_(TE/LE)) represent thesignal-plus-noise in two measurements, the following represents theaverage signal plus total noise level over two measurements:

AVG_(TE/LE)=1/2×Sum_(BITS)

Thus, the desired measurement is completed by taking the sum

OUTPUT=/LE′ _(BITS) +TE′ _(BITS)−AVG_(TE/LE)

and rounding up any fractional remainder to the nearest whole number, asshown in FIG. 6. In this expression, the total noise level and theaverage signal level are both subtracted from the sum of the rawmeasurements, resulting in a substantial reduction in noise levels inthe output. It will be seen that any number of samples of /LE and TE maybe taken (and processed in similar ways) to improve the measurementaccuracy. Further, various techniques, such as averaging the multiplesamples, where appropriate, can further improve accuracy.

It is apparent that the discrete nature of the output lends itself tothe many systems for and techniques of signal processing andinterpolation available in current electronic and computing systems. Anadvantage of the present invention is that these systems and techniquesmay be applied more easily than in other prior art methods and systemssince in most cases the binary output will be generated very quickly,while the output data will be sampled or otherwise read by the system ata much slower rate. This allows tens or hundreds of clock cycles forsignal processing (averaging, interpolation, transforms, etc.) for eachoutput measurement provided by the comparison circuit. For example, anautomotive collision warning system using the present method mightrequire only 100 nanoseconds to make enough distance measurements todetermine a highly accurate average value, yet the vehicle may requirethe value to be presented to the control system only once everymillisecond. Thus, in accordance with one aspect of the present systemand method would require a significantly small portion, e.g., only0.01%, of the control system's processing time to provide themeasurement, while the rest may be used for other purposes.

Nested Bit Patterns

In accordance with one aspect of the invention, a novel method (andsystem employing the method) is proposed in which a bit stream, orpattern of bits, is ‘nested’ within a longer pattern of bits, whichitself is nested within a longer pattern of bits, etc. The concept ofthis aspect of the invention is illustrated by way of example in FIG. 7.The application of this concept to the system for and method of signaldelay measurement disclosed herein greatly enhances the ability of themethod of measuring delays that span several orders of magnitude intime. For instance, an automobile cruise control system measuring thedistance to another vehicle may need to measure distances of severalthousand feet to operate safely at highway speeds. The same system,however, may need to measure accurately at distances as small as onefoot in parking or other low speed situations. For many prior artsystems, the requirement to perform over three orders of magnitude indistance is a significant challenge.

The method of nested bit patterns is described as follows. Referring toFIG. 7 a, a bit sequence is shown which represents the shortest durationmeasurements to be made, or T₂=SL_(B). A repeated pattern of this bitsequence is generated, and on a longer time scale appears as shown inFIG. 7 b. This sequence is also repeatedly generated, and on an evenlonger time scale appears as shown in FIG. 7 c, and once more in FIG. 7d. This process may be repeated indefinitely, until the time scalematches the desired upper bound of delays to be measured, or T_(S).While the figure shows ‘nesting’ to three orders of magnitude, whereT₂=T_(S)/10,000, even higher levels of nesting are achievable.

The interpretation of the nested bit patterns is substantially the sameas for a normal bit pattern, except that the clock speeds are scaledaccordingly for each iteration. In FIG. 7, the transmitted bit length T₂is arbitrarily set to be one-tenth of the shortest time scale ofinterest. The time scale for FIG. 7 a is T_(S)/1000, requiring logicalcomparison configuration circuitry which can capture return bits oflength T₂/S=T_(S)/10000 or shorter. The time scales for the example inFIGS. 7 b, 7 c, and 7 d are T_(S)/100, T_(S)/10, and T_(S)/,respectively, but in practical systems may be shorter or longer. Somesmoothing, or time-averaging, of the longer time scale bit streams maybe necessary to accurately capture the signal envelope, but may beperformed in ways which are well known to those skilled in the arts.

Use of this novel approach enables the delay time measurement of thepresent invention to be made across several orders of magnitude at thesame time. That is, it will be apparent to those skilled in the artsthat the logical comparison configuration can include multitude ofcomparison circuits or configurations, operating in parallel, but atdifferent clock speeds, can produce a delay time measurement with anextremely broad dynamic range. Importantly, in accordance with oneaspect of the invention, the length of time the system and methodrequires to take a measurement across the full dynamic range isproportional to the longest time scale used, and increased precisiondoes not require longer time periods for the averaging of a large numberof bits. Instead, the precision comes from the interpretation of thenested bit streams already contained within the longer stream, whichdoes not take any additional time.

Hardware Implementations

The following paragraphs describe several preferred embodiments of thesystems for accomplishing the foregoing. Others may be readily createdby those skilled in the arts.

FIG. 8 shows a transceiver pair 80 a and 80 b in communication with eachother according to the present invention. In this embodiment, thedevices are substantially the same.

Signal generation in the source transceiver is initiated by a localprocessor 81 a, and is followed by modulation using the modulation unit82 a and transmission of the signal from antenna 83 a through thetransmission medium, represented by the signal channel “CH1”. Thetransmitted signal will typically be composed of a carrier frequency (orfrequencies), provided for example by the source of the carrier signalindicated at 84, upon which is modulated (by unit 82 a) the originaldigital signal. In some cases, the carrier will only be present when thesignal is present (for example, On-Off-Key modulation, or OOK), or itmay be present continuously in forms corresponding to other modulationtechniques, such as amplitude, frequency, pulse-code, or phasemodulation. It may also be present intermittently, for example as inspread spectrum, CDMA, or other common modulation techniques.

A small part of the modulated source signal is sampled, or split fromthe output of the modulation section, and routed using acoupler/splitter unit 85 a to a detector circuit 86 a. The detectorcircuit may simply detect the presence of the carrier(s) as in OOKmodulation, or it may perform a more complicated task, such as partialor complete demodulation to determine the actual beginning of thedigital signal. In any case, the detection function produces a ‘flag’signal, such as a digital bit rising edge followed after a known time bya falling edge. This flag marks the departure time of the outgoing bitor bit stream, whether that corresponds to the carrier departure time ornot.

The detector circuit is followed by a high speed comparison circuit 87 apreferably composed primarily of high-speed logical function blocks andgates. This circuit 87 a performs the “comparison” function describedabove. The signal departure flag is held, or latched, in the logicalcomparison circuit for later use in the process.

The modulated signal travels to the destination, and is received by thedestination transceiver (in the illustration shown, medium interface 83a to medium interface 83 b, or vice versa), each medium interface beingfor example, an antenna, transducer, light source, etc. As before, partof the received signal is sampled, or split off, using coupler/splitter85 b and routed through a detector 86 b to a circuit 87 b of high-speedlogical function blocks and gates substantially similar to that in thesource transceiver. The signal arrival flag is latched, as before, andthe signal is simultaneously routed to the demodulation section 88 b.The demodulated signal can be processed by destination processor 81 b torecover source transceiver commands and execute them accordingly, or toprovide other functions. Importantly, this permits the method and systemdisclosed to function in conjunction with, and without significantadditional resources to, industry standard communication protocols suchas IEEE 802.11x, 802.15x, WiFi, GPOS, etc. The processor will generate areturn signal, which may be identical to the original or it may bechanged to include information to be transmitted back to the sourcetransceiver. A switching unit is provided at 89 a and 89 b for switchingbetween the modulation and demodulation modes of operation depending onwhether the transceiver is transmitting (where the switching unit is setfor modulation), or receiving signals (where the switching unit is setfor demodulation) over the communication channel CH1.

It should be noted that for the purposes of measurement accuracy, thedelay introduced by the destination processor might be large compared tothe round trip transit time to be measured. Processing should thereforebe designed to minimize the time taken to generate the response, and todo so in a predictable manner. One solution to this problem is torequire an arbitration process for granting the transceivers permissionto transmit on CH1. Once arbitration is complete, the source transceivercan transmit a short burst of data to the destination transceiver, andthis burst is used for the comparison. Processing time at thedestination transceiver can be minimized in this case.

The return signal generated by destination processor 81 b is modulatedby modulation unit 82 b onto the carrier frequency, and routed to themedium interface 83 b. The departure flag of the return signal producedby detector 86 c is latched into the logical comparison circuit 87 b. Atthis point, the logical comparison circuit in the destinationtransceiver will have obtained two time flags, and can logically comparethem according to the method described previously to produce an outputwhich is readable by the destination processor. The output (denotedT_(DEST)), corresponds to the propagation time of the signal through thecomponents of the destination transceiver, and thus indicates the delayerror the destination transceiver introduces. This data can betransmitted back to the source (following the initial return signal) foruse by the source transceiver in removing errors from the measurement.

The modulated return signal travels back to the source over CH1 and isreceived by the source transceiver 80 a. Once again, part of thereceived signal is sampled, or split off, and routed to the detector 86d and logical comparison circuit 87 a. The signal arrival flag islatched into the logical comparison circuit 87 a, and the signal issimultaneously routed to the demodulation section 88 a. The sourcetransceiver logical comparison circuit 87 a now has two time flags, andcan compare them to produce an output which is readable by the sourceprocessor 81 a. In this case, the output (denoted TTOT) corresponds tothe propagation time of the signal through the transmission mediuminterfaces, the transmission medium (there and back), and the componentsof the destination transceiver.

One final measurement is needed, which can typically be generated inadvance and stored in the source processor's memory, to be updated atregular intervals. A signal is generated by the source transceiver, andmodulated as before. However, this ‘self-test’ signal is immediatelyrouted from the modulation section 82 a through switching unit 89 a tothe demodulation section 88 a (the path may include the mediuminterface(s) if these components introduce substantial errors). Asbefore, the difference between departure and arrival times of theself-test signal, as determined by the logical comparison circuit, isread by the source processor 81 a. This measurement, denoted T_(SOURCE),corresponds to the propagation time of the signal through the sourcetransceiver components.

Thus, the total propagation time of the signal to the destinationtransceiver and back (T_(TOT)) is composed of three elements:

T _(TOT) =T _(SOURCE)+2T _(MEDIUM) +T _(DEST)

T_(TOT), T_(SOURCE) and T_(DEST) have all been measured, as discussedabove. Therefore, T_(MEDIUM) can be calculated directly:

T _(MEDIUM)=1/2[T _(TOT) −T _(SOURCE) −T _(DEST)]

In this way, the invention can determine the propagation time of adigital signal as it makes a round trip from source transceiver, througha medium to a destination transceiver, and back. The time measurementcan be directly converted to a distance measurement with standardtechniques, if the speed of propagation of the signal is known for eachmedium the signal traverses.

Duplex Operation

The system of FIG. 8 uses one communications channel (CH1) for signalstraveling back and forth. This can be referred to as ‘simplex’operation, and is adequate for situations where the measured delayexceeds the duration of the transmitted bit or bit stream, as would bethe case if T1 were greater than T2 in FIG. 1. However, in manyapplications this will not be the case. In fact, it is the moredemanding situations, where T2 is greater than T1, which one aspect ofthe present invention seeks to address.

FIG. 9 shows an embodiment of the present invention, which uses separatecommunications channels, CH1 and CH2, for the transmitted and returnsignals, referred to as duplex operation.

In a manner substantially similar to that described above for simplexoperation, the transmitted bit or bit stream is transmitted by thesource, traverses the medium, is received at the destination, andreturns through the medium to the source. At each stage, as before, thearrival and departure flags are latched by the logical comparisonconfigured circuits, and the round trip delay time is measured. However,this embodiment makes use of channel CH1 for transmitting the initialsignals from transceiver 90 a to transceiver 90 b, and additionalinterfaces 91 a and 91 b and a second channel CH2 for the return signals(as seen in FIG. 9). This eliminates the need for switching unit 89.This has the advantage of allowing the return signal to overlap thetransmitted signal in time, so that a delay measurement can be made in atime period less than one bit length (i.e., less than T2). Thus, delaysof arbitrarily short duration may be measured, with a suitable choice ofthe over-sampling factor, S.

Modulation/Demodulation Bypass and Error Correction

FIG. 10 illustrates another embodiment of the present invention thatprovides another aspect of the disclosed system for and method ofminimizing the measurement errors that would otherwise be introduced bythe destination transceiver and any intermediate transceivers. This isparticularly valuable for signals traversing more than one transceiverbefore returning to the source transceiver, as in so-called ‘multiplehop’ communications in packet transfers, mesh networks, ad-hoc and soon. In these cases, prior art methods which rely on time-tagging, edgedetection, or time averaging of a large number of transmissions, willsuffer significant errors due to the delays introduced by thedemodulation and modulation circuitry in each transceiver. These delayscan be many times larger than the delays due solely to the distancebetween transceivers, and the problem becomes even more challenging whenthe distances are short and the time delays are correspondingly short.

In FIG. 10, a duplex transceiver pair 100 a and 100 b is shown with anarrangement of switching units 101 a, 101 b, 101 c, and 101 d allowingeach transceiver to route the output from the modulation section eitherto the appropriate medium interface 102 (e.g., antenna, transducer,light source, etc.) or to a frequency converter 103 which changes thecarrier frequency from F1 to F2. Note that the switching units arearranged such that the modulated output can be transmitted on either CH1or CH2, and the signal from either medium interface can be routed to thedemodulation section, after appropriate conversion to F1 for signalsreceived on CH2.

In a typical measurement, signal generation in the source transceiver,indicated as 100 a in FIG. 10 would occur as described previously. It isassumed that the arbitration process has been completed, and themeasurement bit or bit stream is being transmitted. As before, themodulated signal would traverse the medium on channel CH1. Thedestination transceiver receives it through medium interface 102 b, butroutes the signal directly to the frequency converter 103 b, whichchanges the carrier frequency to F2. No further processing of the signalis required. The signal is immediately routed to the medium interface102 c and retransmitted on CH2. The source transceiver receives itthrough medium interface 102 d, and performs the logical comparison todetermine the delay time, as described above. This measurement willinclude only the errors that are due to the frequency converter circuit,which are significantly smaller than those which demodulation,processing, and re-modulation would have introduced. The sourcetransceiver may perform a self-test, as discussed previously, as can thedestination transceiver, to further characterize (and later correct)delay errors introduced by the transceiver components and mediuminterfaces.

Distance Measurement and Positioning Applications

If accurate distance measurements are made between multipletransceivers, and the measurements are shared among those transceivers,then the angular position of the transceivers with respect to oneanother can be determined through triangulation techniques.Specifically, if source and destination transceivers are both positionedat known locations, and their spatial separation remains known, thenboth the distance and angular position of a third transceiver (relativeto the first two) can be determined by the system, as illustrated inFIG. 11. Two transceivers configured this way will be referred toherein, for illustrative purposes only, as ‘reference transceivers’. Itwill be apparent that a reference pair may also be composed of a singletransceiver with two medium interfaces, such as a radio transceiver withtwo antenna inputs, a duplex transceiver, or an ultrasound transceiverwith two ultrasound transducers. A ‘reference pair’ may be composed ofany two transceiver types. For example, a system deployed on largemobile machinery may employ one primary transceiver, which may be fullynetworked with other vehicle systems, along with a simpler, lower costdevice placed a known distance away on the same machinery. An embodimentof the present invention capable of determining relative angularposition of a plurality of transceivers is described in the followingparagraphs.

Readers and Tags

Although there may be many applications and utilities, the abovedescribed system and method has particular application and utility withrespect to radio frequency identification systems and methods. RadioFrequency Identification (RFID) technology (passive or active) can beincorporated into any transceiver. The present disclosure does notassume any particular RFID system or standard, and can function withoutit. Even without RFID data the system will be able to accumulateknowledge of how many destination transceivers are in the field of view,and where they are with respect to the reference transceivers (distanceand azimuth angle). The addition of RFID capability will enable thesystem to determine what type of objects are represented by thedestination transceivers (through interpretation of ID data) allowingthe system to process alarms, warnings, and corrective actionsaccordingly.

Transceivers may take the form of small ‘Tags’, indicated in FIG. 11 at110, which are affixed to items to be tracked or protected such aspersonnel, shipping pallets, or structural elements. Such Tags wouldhave minimal components and power consumption, to allow low costmanufacture and long battery life. Other, more capable transceiversmight be affixed to larger items such as heavy equipment, fixedstructures, ships, automobiles, mobile machinery, or otherinfrastructure. These may be referred to as ‘Readers’, indicated in FIG.11 at 111 in accordance with RFID system terminology. Readers arespecifically designed to produce meaningful output to a human operator,or a network providing the exchange of data and commands to and from thetransceivers. As such, they will often have network communicationinterface circuitry.

FIG. 12 illustrates an example of an arrangement of transceiversconfigured as Readers and Tags, in an architecture which can comply withstandard RFID industry practice. The figure shows a number of Tags 120,which are assumed to be inexpensive, small, self-powered, and highlymobile. Readers 121 are shown on mobile platforms, but are connected viaa wireless network 122. The network communications are managed by theReaders themselves. Each will recognize the entry or departure of anyother. Distance measurements to all in-range Tags will be shared amongthe Readers. The Readers will also measure the distance betweenthemselves and all other in-range Readers. Azimuth angle determinationbetween Readers and other Readers, and between Readers and Tags, isdisclosed in the following paragraphs.

Triangulation

An example of a method of triangulation using the signal delaymeasurement methods described herein is illustrated using thedefinitions shown in FIG. 13. Transceivers are located at points O, Pand Q. Arbitrarily, O and Q are assigned to be a reference pair, i.e.,they are comprised of a single source transceiver with two mediainterfaces, or two source transceivers. It is assumed that the distanceC between them is known, and remains fixed, and that an imaginary linejoining them makes a known angle with the mobile platform on which theyare both mounted. This means that determination of the angles α and β,along with either length A or B, will uniquely define the position ofthe transceiver at P, with respect to the mobile platform.

The unknowns may be calculated as follows. The Law of Cosines applies toeach side of this triangle, so that:

C ² =A ² +B ²−2AB Cos(γ)  (1)

B ² =A ² +C ²−2AC Cos(β)  (2)

A ² =B ² +C ²−2BC Cos(α)  (3)

A and B will be provided by the transceivers themselves. In fact, thetransceiver at O will deter mine A and C, the transceiver at P willdetermine A and B, and the transceiver at Q will determine B and C, eachusing the signal delay measurement method of this invention, so thatverification of the measurements and/or averaging or other errorcorrections may be applied. The algorithms and calculations can becarried out on the processor of the transceiver (112 in FIG. 11), or aseparate processor in the transceiver, or separate from the transceiver.Since A, B, and C are now known, all three of the angles can becalculated using the equations below:

Cos(γ)=(A ² +B ² −C ²)/2 AB  (4)

Cos(β)=(A ² +C ² −B ²)/2 AC  (5)

Cos(α)=(B ² +C ² −A ²)/2 BC  (6)

Two-dimensional azimuth angle measurements for the destinationtransceivers may be obtained using two methods. The first method, usinga single source transceiver on each mobile platform, uses two or moremedia interface sections for each source transceiver. For a system usingradio signals through air, for example, this can comprise two or moresource transceiver antennas or two or more separate source transceiverunits located at known, fixed distances from each other on the samevehicle. Either simplex or duplex communication may be used, asdescribed earlier, however the dual-frequency nature of duplexcommunications lends itself readily to the use of multiple antennas.

Another example of a method and system involves multiple sourcetransceivers (one or more per mobile platform) operating in the samearea as seen in FIG. 12. In either case, measurements for a giventransceiver, from a given reference pair, will be compared with similarresults from nearby transceivers in a process of real-time verificationand error reduction. It will be apparent that use of three transceiversin a reference set will allow three dimensional positioning in a methodsubstantially similar to that shown for two dimensional positioning.

Network Formation and Data Sharing

The ability of the system and method to perform the azimuth anglemeasurement function depends primarily on rapid and accurate distancedetermination between destination transceivers and source transceivers,but also on the systems' ability to coordinate the acquired distancemeasurements and identification data between source transceivers. Thiscan be managed over an ad hoc wireless network, indicated by way ofexample at 122 in FIG. 12, established between the source transceivers.Coordination over this ‘command and control’ network will allowcontinual refinement of both distance and azimuth angle measurementswhen more than one source transceiver is operating in an area, as wellas ‘traffic management’ of the higher speed channels (CH1 and CH2).These features will tend to improve overall measurement accuracy incrowded work areas, rather than degrade it. Referring again to FIG. 12,the distance measurement data paths are shown as dashed arrows, and thelower speed command and control paths are shown as solid arrows.

It is anticipated that the methods and systems disclosed herein will bedeployed in many applications using multiple mobile platforms whicharrive and depart in an unpredictable manner. Such might be the case inconstruction, mining, agricultural, and materials handling environments,boating, highway, and emergency location applications, among others. Thecapability for multiple transceivers to communicate and share distanceand azimuth angle measurements as they approach and operate near eachother is a substantial advantage of the present invention. A typicalscenario might involve 3 or 4 vehicles and 10 pedestrian workers withina work area, with their associated source transceivers and destinationtransceivers (Readers and Tags). These devices will detect the presenceand range information of all the others, and azimuth angles as well,allowing each transceiver to make appropriate warnings or take otheractions. By enabling the formation of ad hoc networks as multipleReaders operate within a common area, using techniques and systemsreadily available in the marketplace, the quality of the distance andazimuth measurements is improved dramatically. Multiple measurements canbe made during each interrogation cycle, and the results shared betweenReaders.

Simple averaging, or more complex analyses well known to those skilledin the arts, may be performed with the data.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A system for making a measurement of the amount of time required forat least one digital bit to traverse a round-trip path from a sourcetransceiver to at least one destination transceiver and back, the systemcomprising: a high speed comparison configuration configured andarranged so as to (a) determine a measurement of the relative timingbetween a transmitted bit produced at a clock rate by the sourcetransceiver and transmitted to a destination transceiver, and acorresponding return bit produced by the destination transceiver andreceived by the source transceiver in response to receiving therespective transmitted bit from the source transceiver, and (b) providein substantially real-time at least one measurement related to orderived from a measurement of the time required for the transmitted andcorresponding return bit to traverse the round trip path.
 2. A systemfor making a measurement of the amount of time required for a digitalbit or bit stream to traverse a round-trip path from a sourcetransceiver to at least one destination transceiver and back, the systemcomprising: a high speed comparison configuration configured andarranged so as to (a) determine a measurement of the relative timingbetween a transmitted bit or bit stream produced at a clock rate by thesource transceiver and transmitted to a destination transceiver, and areturn bit or bit stream produced by the destination transceiver andreceived by the source transceiver in response to receiving thetransmitted bit or bit stream from the source transceiver, and (b)provide in substantially real-time at least one measurement related toor derived from a measurement of the time required to traverse the roundtrip path, wherein the high speed comparison configuration includeserror correction configured and arranged so as to measure and correctfor errors.
 3. A system according to claim 2, wherein error correctionis configured and arranged so as to correct for one or more of thefollowing: fixed delays and jitter.
 4. A system according to claim 1,further including a sampling configuration configured so as to sampleeach transmitted bit and the corresponding return bit at a multiple ofthe clock rate of the transmitted bit prior to comparing the relativetiming of the two.
 5. A system according to claim 1, further including aprocessor configured and arranged so as to determine the distancebetween the source transceiver and the destination transceiver as afunction of the time required to traverse the round trip path.
 6. Asystem according to claim 1, further including a processor configuredand arranged so as to determine the two dimensional azimuth angleposition of the source transceiver with respect to the destinationtransceiver and one other transceiver.
 7. A system for determining thedistance between two transceivers in substantially real time using ameasurement of the delay between (a) transmission at a clock rate of atransmitted digital signal by one of the transceivers functioning as asource transceiver and the other of the transceivers functioning as adestination transceiver and (b) reception of a return digital signaltransmitted by the destination transceiver to the source transceiver inresponse to receiving the transmitted digital signal, the systemcomprising: the source and destination transceivers configured andarranged so as to define at least in part a round trip transmission pathof the transmitted digital signal and the return digital signal: thesource transceiver including an oversampling component configured andarranged so as to oversample the return digital signal at anoversampling rate S so as to provide an oversampled return signal,wherein the oversampling rate S is a multiple of the clock rate of thetransmitted digital signal; a high speed comparison configurationconfigured and arranged so as to compare the transmitted digital signalto the oversampled return digital signal and generating at least twobytes of bits at the oversampling rate, wherein one of the bytescorresponds to a measurement of a differential time period during whicha leading edge of the transmitted digital signal is transmitted and aleading edge of the return digital signal is received by the sourcetransceiver, and the other of the bytes corresponds to a measurement ofthe differential time period during which a trailing edge of thetransmitted digital signal is transmitted and a trailing edge of thereturned signal is received by the source transceiver; a valueassignment component configured and arranged so as to assign a value toeach of the bytes as a function of an ordering of the bits of each ofthe bytes, wherein the sequence of the least significant bit to mostsignificant bit of one byte is temporally reversed; and a delaydetermination component configured and arranged so as to determine thedelay as a function of the assigned values of the two bytes.
 8. Thesystem according to claim 7, further including a processor configuredand arranged so as to accumulate a plurality of assigned valuesrepresenting the delays for a plurality of transmitted and returnedbits.
 9. The system according to claim 7, further including a noisecompensation component configured and arranged so as to compensate fornoise in the return digital signal.
 10. The system according to claim 9,wherein the noise compensation component is configured and arranged soas to compensate for an out-of-phase component of the noise.
 11. Thesystem according to claim 10, wherein the noise compensation componentis configured and arranged so as to determine an out-of-phase componentas a function of the difference between a logical inverse of one of thebytes to a logical value of the other of the bytes.
 12. The systemaccording to claim 8, further including a noise compensation componentconfigured and arranged so as to compensate for noise in the returndigital signal.
 13. The system according to claim 12, wherein the noisecompensation component is configured and arranged so as to compensatefor an out-of-phase component of the noise.
 14. The system according toclaim 13, wherein the noise compensation component is configured andarranged so as to determine the out-of-phase component of noise as afunction of the difference between a logical inverse of one of the bytesto a logical value of the other of the bytes.